configure: Use string for tm_zone assignment

This matches what the actual sources do.  Clang 16 and GCC 14
no longer support converting ints to pointers implicitly, so the
configure probe always fails with these compilers.

configure.ac

@@ -129,7 +129,7 @@ AC_CACHE_CHECK(for tm_zone in struct tm, ac_cv_struct_tm_zone,
       #include <time.h>
     ], [
       struct tm tm;
-      tm.tm_zone = 1;
+      tm.tm_zone = (char*)"UTC";
     ])],
     [ac_cv_struct_tm_zone=yes],
     [ac_cv_struct_tm_zone=no]